KR102353270B1 - Device for limiting voltage on DC voltage networks - Google Patents

Abstract

The present invention relates to a device for limiting the voltage on a DC voltage network 1 , in which overvoltages due to switching operations occur at the first supply potential level of the DC voltage network 1 ( 2) and the second supply potential level (3). Device 10 comprises at least two limiter cells 10-1, ..., 10-n connected in series between a first supply potential level and a second supply potential level, such at least two limiter cells 10-1, ..., 10-n Each of the base cells 10-1, ..., 10-n includes a controllable switching element 11-1, ..., 11-n, a discharge resistor 12-1, ..., 12-n. and an arrangement of a capacitor (13-1, ..., 13-n), over these at least two limiter cells (10-1, ..., 10-n), a first supply potential level ( _{The voltage U ges} applied between 2) and the second supply potential level 3 drops. During operation of the device, the voltage drops U1, U2, U3 across capacitors 13-1,..., 13-n of certain limiter cells 10-1,..., 10-n Based on this, the controllable switching elements 11-1, ..., 11-n of the limiter cell are switched on or switched off.

Description

Device for limiting voltage on DC voltage networks

The present invention relates to a device for limiting the voltage on a DC voltage network, in which overvoltages as a result of switching operations are equal to the first supply potential level of the DC voltage network and It may occur between the second supply potential level.

Overvoltages due to switching operations in DC voltage networks occur, for example, during feedback of energy from electrical machines or as a result of switching operations in inductive DC voltage networks. Overvoltages must be limited to prevent damage to components connected to the DC voltage supply network.

In addition to varistors and RC-networks, electronic voltage limiters, so-called brake choppers, are known to be used as surge arresters. Although varistors can absorb high discharge currents very well, these varistors are not suitable for frequent overvoltages such as those encountered during switching operations. When subjected to frequent energy absorption, varistors tend to age very quickly. Capacitors used in RC networks must be sized according to expected overvoltages. Capacitors are expensive if large amounts of energy have to be absorbed. In addition, the capacitor has a significant size and considerable weight. Brake choppers are used on DC link circuits of converters. Brake choppers are used to dissipate the braking energy of an electric machine connected to a DC voltage supply network.

An example of such a brake chopper known from the prior art is shown in FIG. 1 . 1 shows a DC voltage network 1 having a first supply potential level 2 and a second supply potential level 3 . Between the first supply potential level 2 and the second supply potential level 3 , a DC-link capacitor 4 is connected. The rectifier on the input side and the inverter on the output side are not shown. A brake chopper having the function of a surge arrester 10 is composed of a series circuit of a controllable switching element 11 and a braking resistor 12 . In this case, the brake chopper or surge arrester 10 is connected in parallel with the DC-link capacitor 4 between the first supply potential level 2 and the second supply potential level 3 . The controllable switching element 11 operates as a switch. The resistor 12 is switched to the intermediate circuit in a pulsed mode by means of a controllable switching element 11 , so that the voltage on the DC-link capacitor 4 is a braking of the electric drive. It again gradually decreases as a result of the voltage increasing due to operation. Thus, the energy fed back by the electric machine during the braking operation until the DC-link voltage that drops across the DC-link capacitor 4 drops below a preset turn-off threshold. converted to heat A corresponding control unit for controlling the controllable switching element 11 is not shown for simplicity.

If energy needs to be dissipated at points in the DC voltage network where there is no capacitance for smoothing, as is the case with the DC-link shown in Figure 1, the brake chopper also has its own small capacitance can have Such a case applies, for example, to the end of a long cable, or to the output of a choke in front of a switching element. An example of this is shown in FIG. 2 . 2 shows a part of a DC voltage network 1 in which a choke 5 and a switching element 6 are connected in series with each other in a conductor of a first supply potential level 2 . A brake chopper forming a surge arrester 10 is connected to a first supply potential level 2 at a node between the choke 5 and the switching element 6 . The other end of this brake chopper is connected in a known manner to the second supply potential level 3 . The brake chopper or surge arrester (10) comprises, in addition to the series circuit of the controllable switching element (11) and the resistor (12), a capacitor (13) connected in parallel to this series circuit, the capacitor (13) can absorb the current generated by the choke 5 in the phases in which the controllable switching element 11 is switched off. In this case, the capacitor 13 is designed only for energy absorption during the switching-off process of the controllable switching element 11 .

A problem with the brake chopper described in FIGS. 1 and 2 is that for higher voltages, in particular voltages exceeding 1000 V, controllable switching elements are very expensive or even not available.
DE 10 2011 053 013 A1 discloses a balancing device for two or more energy storage units connected in series. The balancing device has a number of by-pass-circuits corresponding to the number of energy storage units for limiting the voltage of the associated energy storage unit, wherein the bypass-circuit is the associated energy storage unit is connected in parallel with and has a series circuit of at least one ohmic resistor and a controllable switch element. If the voltage level for the associated energy storage unit exceeds an acceptable limit, the control device switches the bypass circuit to redirect the current flow to the bypass circuit and to discharge the energy storage unit to acceptable voltage values. Activate the switch element.

It is an object of the present invention to specify a device for limiting the voltage for a DC voltage network whose function and/or design is improved.

This object is achieved by a device according to the features of claim 1 . Advantageous embodiments are obtained from the dependent claims.

The invention creates a device for limiting the voltage on a DC voltage network, in which overvoltages as a result of switching operations can occur between a first supply potential level and a second supply potential level of the DC voltage network. have. The apparatus includes at least two limiter cells interconnected in series between a first supply potential level and a second supply potential level. Each of the limiter cells includes an arrangement of a controllable switching element, a discharge resistor and a capacitor. The (total) voltage applied between the first supply potential level and the second supply potential level is connected in series It is dropped across at least two limiter cells. In operation of the device, depending on the voltage dropped across the capacitor of the respective limiter cell, the controllable switching element of this individual limiter cell is switched on or off. The device comprises a control unit for activating individual switching elements of the limiter cells. The control unit is configured to operate the controllable switching elements in a pulsed mode with a time offset when an overvoltage occurs between the conductor of the first potential supply level and the conductor of the second potential supply level. is composed The voltages of the capacitors are determined and compared, and at a given time, the controllable switching element of one of the limiter cells, comprising the capacitor with the highest voltage currently applied across it, is switched on while the other limiter cell is switched on. Their controllable switching elements are switched off.

A device for limiting voltage (hereinafter also referred to as voltage limiter or surge arrester) proposed according to the invention is a plurality of interconnected in series circuit between a first supply potential level and a second supply potential level. It is designed in a modular way with limiter cells of By means of a series connection of a plurality of limiter cells, only a partial voltage across the switched-off controllable switching elements is dropped, so that low voltage controllable switching elements may be used compared to a single limiter cell. can This allows less expensive controllable switching elements to be selected.

Another advantage is that, due to the presence of a capacitor in each limiter cell, the voltage on the individual controllable switching element of the limiter cell cannot change suddenly. As a result, the balancing of the individual controllable switching elements of the limiter cells, ie the precise temporal control, is not time critical.

The maximum voltage dropped across each of the limiter cells can be regulated independently of the other limiter cells. This means that, if possible, a higher level control unit is not required. Such a higher level control unit is convenient, for example, if the voltage occurring between the first supply potential level and the second supply potential level needs to be set to a variable level.

Therefore, in the operation of the device for limiting voltage, at a given time, the controllable value of the limiter cell in question comprising the capacitor having the highest voltage applied across it compared to the voltages of the capacitors of the remaining of the limiter cells. It is provided that the switching element is switched on. In other words, the voltages across the capacitors of all limiter cells are determined separately and compared to each other. The switching element of the limiter cell in question is then switched on, which includes the capacitor across which the highest voltage drops. This ensures that, even at high overvoltages, an overload of the controllable switching element of the individual limiter cell cannot occur.

A further advantageous embodiment provides that, in the operation of the device, the controllable switching elements are operated in a pulsed mode. This means that the individual limiter elements are operated in a pulsed mode (“chopped up”) to gradually lower the voltage between a first supply potential level and a second supply potential level, as is the case in a conventional brake chopper. )") means. In particular, for the device for voltage reduction, proposed according to the invention, it is possible to use known control units of conventional brake choppers.

A further advantageous embodiment is that, in the operation of the device for limiting voltage, at a given time, a first subset of the controllable switching elements of the at least two limiter cells are switched on, and the at least two limiter cells are switched on. provide that a second subset of the controllable switching elements of In such a design, it is ensured that simultaneous activation of all limiter cells (ie simultaneous activation of the individual controllable switching elements of all limiter cells) is possible, even in case of a short circuit in which the maximum discharge current has to be dealt with. This can in particular ensure that no damage to the components of the limiter cells can occur, even in case of short circuits and consequent high discharge currents.

A further advantageous embodiment provides that the arrangement comprises a series circuit of a controllable switching element and a discharge resistor, with a capacitor connected in parallel with this series circuit. In such a configuration, the controllable switching element may be an Insulated Gate Bipolar Transistor (IGBT).

In another embodiment, the series circuit may include an inductor in addition to the controllable switching element and the discharge resistor. The size of the inductor is advantageously selected according to the size of the capacitor and/or the size of the discharge resistor in such a way that an RLC resonant circuit is formed by each limiter cell. This makes it possible to select a thyristor as a controllable switching element, since by means of the RLC resonant circuit, an automatic shut-off, ie switching off, of the thyristor is possible. When the controllable switching element is designed as a gate turn-off (GTO) thyristor or the thyristor is provided with a quenching circuit, simplified control is obtained.

A further advantageous embodiment provides that the number of limiter cells exceeds two. The exact number of limiter cells depends on the nominal voltage between the first supply potential level and the second supply potential level and the voltage breakdown strength class of the controllable switching element used in the individual limiter cells. In principle, the functionality is already guaranteed if two limiter cells are connected in series between a first supply potential level and a second supply potential level in turn.

The device for limiting the voltage comprises a control unit for activating individual switching elements of the limiter cells. As already explained above, the control unit is designed such that it carries out a higher level adjustment of the temporal switching on and switching off behavior of the controllable switching elements of the different limiter cells. can be However, the control unit can also be designed in such a way that it performs the control of the exclusively controllable switching elements in dependence on the voltage applied across the capacitors of the limiter cells.

Additionally, it is advantageous if the nominal voltage between the first supply potential level and the second supply potential level of the DC voltage network exceeds 1000 V. In other words, this means that a device for limiting the voltage is provided in particular in medium-voltage networks, for example used for ships or in industrial plants.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is explained in more detail below on the basis of exemplary embodiments of the drawings. The following is shown:
1 is a schematic representation of a known DC-link circuit of a DC voltage network, with a device for limiting the voltage, designed as a brake chopper;
2 is a schematic representation of a known brake chopper at the output of a choke in front of the switching element;
3 shows a schematic representation of a first design variant of a device for limiting voltage according to the invention for a DC voltage network;
4 shows a schematic representation of a second design variant of a device for limiting voltage, according to the invention;
In the drawings, equivalent elements are labeled with the same reference numbers.

3 shows a schematic representation of a part of a DC voltage network 1 , in which one conductor of a first supply potential level 2 and one conductor of a second supply potential level 3 are shown. Between a conductor at a first supply potential level 2 and a conductor at a second supply potential level 3 , a device 10 for limiting a voltage according to the invention (hereinafter referred to as a voltage limiter or surge arrester) ) is connected. The surge arrester 10 connects a plurality of (n) limiter cells 10-1, ..., 10-n in series between the first supply potential level 2 and the second supply potential level 3 . include In this case, this number n is two or more. The number of limiter cells 10-1, ..., 10-n is in each case designed the same, in each case the controllable switching element 11-i, the discharge resistor 12-i and the capacitor Including the arrangement consisting of (13-i), where i = 1 to n. In each of the limiter cells 10-i, the controllable switching element 11-i and the discharge resistor 12-i are connected in series with each other. A capacitor 13-i is connected in parallel with this series circuit of the controllable switching element 11-i and the discharge resistor 12-i.

The node between the discharge resistor 12-1 and the capacitor 13-1 of the first limiter cell 10-1 is connected to a conductor at a first supply potential level 2 . The node between the load terminal of the controllable switching element 11-1 of the first limiter cell 10-1 and the capacitor 13-1 is the discharge resistor of the immediately following second limiter cell 10-2. It is connected to the node of (12-2) and the capacitor (13-2). Between the main terminal of the controllable switching element 11-n and the capacitor 13-n, the node of the last limiter cell 10-n is connected to a conductor of a second supply potential level 3 . In the limiter cells 10-i, IGBTs are provided as controllable switching elements 11-i. The control terminals of the IGBTs are connected to a control unit, not shown.

_{The total voltage U ges} , which drops between the conductor of the first supply potential level 2 and the conductor of the second supply potential level 3 , depends on the number of limiter cells n at the partial voltages U _i ,. .., U _n ), where each partial voltage U _i is dropped across the respective limiter cells 10-i.

In the event of an overvoltage occurring between the conductor at the first supply potential level 2 and the conductor at the second supply potential level 3 , this overvoltage is gradually dissipated by means of the voltage limiting device 10 . For this purpose, the controllable switching elements 11-i of the limiter cells 10-i are operated in a pulsed mode with a time offset. In accordance with the operation of the brake chopper, in each controllable switching element 11-i that is switched on, a current is transferred through the associated discharge resistor 12-i and converted into heat. When the associated controllable switching element 11-i is switched off, voltage buffering via the associated capacitor 13-i of the individual limiter cell 10-i follows, for the reason Because current can now flow into capacitor 13-i.

The topology of the series circuit of limiter cells and the arrangement of limiter cells is such that, in each limiter cell, the voltage on the individual IGBT cannot change abruptly and thus a balancing operation, i.e. all limiter cells 10- It ensures that no temporal synchronization of the switching on and switching off behavior of the controllable switching elements 13-i of i) is required. This is made possible by a capacitor provided in each of the limiter cells 10-i.

The individual limiter cells can regulate their maximum voltage independently of one another, no higher level adjustment by the control unit is required to control the controllable switching elements 11 - i . However, for example, if a variable total voltage U _ges has to be set, a higher level of adjustment is possible.

One advantage of the design described in FIG. 3 lies in the fact that low voltage classes, ie IGBTs up to 1200 V, can be used as controllable switching elements, assuming _{a nominal total voltage U ges in excess of 1000 V.}

The series connection of the capacitances of the capacitors 13-i in the limiter cells 10-i is, if appropriate, of the conductor at the first supply potential level 2 and the second supply potential level 3 . It may be in parallel with a DC-link capacitor connected between the conductors. As a result, the capacitances of the capacitors 13 - i can at least partially replace the function of the DC-link capacitor, and thus the DC-link capacitor can be dimensioned smaller. Then, in such a configuration, as schematically illustrated in FIG. 1 , the device 10 for limiting the voltage and the DC-link capacitor can be considered as a unit.

When using IGBTs as the controllable switching elements 11-i, it is important to take into account the fact that in case of a fault such as a short circuit, these IGBTs can only be overloaded by two to three times the rated current. Therefore, the design or selection of the number n of limiter cells 10-i is advantageously such that all limiter cells 10-i This is done in such a way that not all controllable switching elements 11 - i of i) need to be switched to the conductive state. If the device 10 for limiting voltage comprises limiter cells, for example n = 2, then at a given time the controllable switching element of one of the cells is switched on and the controllable switching element of the other limiter cell is switched on. is switched off. For a large number (n) of limiter cells 10, a different partitioning may be selected. The IGBT of the limiter cell 10-i including the capacitor 13-i having the highest voltage applied across it compared to the voltages of other capacitors of the other limiter cells of the limiter cells 10-i is switched In the manner in which they are turned on, the IGBTs are switched on and off. This automatically and incidentally causes pulsed switching on and switching off of the individual controllable switching elements 11 - i .

Then, in case of a short circuit in which a maximum discharge current can occur, all of the controllable switching elements or IGBTs 11-i are switched on at the same time. This allows voltage overshoots to be dissipated as quickly as possible without causing damage to the switching elements.

To handle higher voltages and higher power levels, thyristors may also be used as controllable switching elements. Such an exemplary embodiment of a device 10 for limiting voltage according to the invention is schematically shown in FIG. 4 . Thyristors have the advantage over IGBTs that they can withstand 10 to 20 times the rated current without breaking down. Since thyristors cannot be switched off (easily), a fine-grained regulation of the voltage is not possible without additional measures. However, with sufficient series connection of m limiter cells, where m preferably exceeds n in the exemplary embodiment according to FIG. 3 , when the thyristors are firing even a single limiter cell Even when fully discharged, the voltage swing is still small.

When thyristors are provided as the controllable switching elements 11-i (where i = 1 to m), in the series interconnection of the controllable switching elements 11-i and the discharge resistor 12-i In addition, an inductor 14-i is provided. The size of the inductor 14-i of each limiter cell 10-i is selected according to the magnitude of the resistance of the discharge resistor 12-i and the capacitance of the capacitor 13-i. Then, as a result, an RLC resonant circuit is formed in each limiter cell 10-i. This means that within limiter cell 10-i, zero current is created to quench individual thyristors 11-i after complete discharge of capacitor 13-i. Thereby, self-quenching of the thyristor is possible.

The number m of limiter cells 10-i when thyristors are used is at least m=3. Given that not all thyristors of all limiter elements are switched on at the same time, as there are only two limiter cells, m = 2, each capacitor 13-i has a first supply potential level (2) and the second supply potential level 3 will need to absorb _{the total voltage U ges .}

In a practical implementation, m = 20 may be chosen as a number, where at a given time, for example, three thyristors of three different limiter cells will be switched on. This over-dimensioning is advantageous, since in this configuration a symmetrical voltage distribution across the individual limiter elements 10-i is not possible.

Claims (13)

A device for limiting voltage on a DC voltage network (1), comprising:
The device (10) comprises at least two limiter cells (10-1, ...) connected in series between a first supply potential level (2) and a second supply potential level (3). , 10-n),
Each of the at least two limiter cells 10-1, ..., 10-n includes a controllable switching element 11-1, ..., 11-n, a discharge resistor 12-1 ,..., 12-n) and an arrangement of a capacitor 13-1,..., 13-n, said limiter cells 10-1, connected in series. _{.., 10-n), the voltage U ges} applied between the first supply potential level 2 and the second supply potential level 3 drops, and during operation of the device, a certain limit _{Based on the voltages U 1} , U ₂ , U ₃ dropped across the capacitors 13-1,..., 13-n of the base cells 10-1,..., 10-n, the the controllable switching element 11-1, ..., 11-n of the limiter cell is switched on or switched off,
The apparatus comprises a control unit for activating the individual switching elements 11-1,..., 11-n of the limiter cells 10-1,..., 10-n. and the control unit is configured in a pulsed mode with a time offset in case an overvoltage occurs between the conductor of the first supply potential level (2) and the conductor of the second supply potential level (3). designed to operate the controllable switching elements 11-1, ..., 11-n in a pulsed mode, wherein the voltage of the capacitors 13-1, ..., 13-n are determined and compared,
In the first mode of operation, the controllable switching element 11-1, ... of the limiter cell comprising a capacitor to which the highest voltage is applied across one of the limiter cells 10-1, ..., 10-n. ., 11-n) is switched on and the controllable switching elements of the other limiter cells (10-1, ..., 10-n) are switched off,
Device. According to claim 1,
In the operation of the device (10) for limiting the voltage, in a second mode of operation, a first subset of the at least two limiter cells (10-1, ..., 10-n) is Controllable switching elements 11-1, ..., 11-n are switched on, and control of a second subset of said at least two limiter cells 10-1, ..., 10-n Characterized in that the possible switching elements (11-1, ..., 11-n) are switched off,
Device. According to claim 1,
The arrangement comprises a series circuit of the controllable switching element (11-1,..., 11-n) and the discharge resistor (12-1,..., 12-n), the capacitors (13) Characterized in that it is connected in parallel with the series circuit of -1,..., 13-n),
Device. 4. The method of claim 3,
Characterized in that the controllable switching element (11-1, ..., 11-n) is an IGBT,
Device. 4. The method of claim 3,
characterized in that the series circuit additionally comprises an inductor (14-1, ..., 14-n),
Device. 6. The method of claim 5,
Characterized in that the controllable switching element (11-1, ..., 11-n) is a thyristor,
Device. 6. The method of claim 5,
Characterized in that the controllable switching element (11-1, ..., 11-n) is a GTO thyristor or a thyristor provided with a quenching circuit,
Device. 8. The method according to any one of claims 5 to 7,
The size of the inductor 14-1, ..., 14-n is such that an RLC resonant circuit is formed in the discharge resistor 12-1, ..., 12-n and/or the capacitor 13- 1, ..., characterized in that selected according to the size of 13-n),
Device. 8. The method according to any one of claims 5 to 7,
Characterized in that the number of limiter cells (10-1, ..., 10-n) is more than two,
Device. 5. The method of claim 4,
It is characterized in that as said controllable switching elements, IGBTs with a voltage class of up to 1200V can be used,
Device. delete delete delete

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